Turf grass (sod) is a living organism that must be handled properly to ensure its survival when it is removed from one location and transplanted to another. Sod is generally harvested using large machinery that cuts slabs of sod from the soil and stacks them on pallets.
FIG. 1 illustrates a portion of a sod harvesting machine 100 that includes a typical cutting head, conveyor system, and stacking system. The cutting head of FIG. 1 includes a chop mechanism 110, an oscillating blade 103, and a roller 104. Chop mechanism 110 is configured to periodically descend into the sod 151 to make vertical cuts defining an initial width/length of the slabs. Oscillating blade 103 oscillates back and forth underneath the sod to sever the slab from the underlying soil 150. Roller 103 applies pressure to the sod as it is cut to facilitate the cutting of clean slabs. Slabs cut by the cutting head are routed to conveyor 101 which lifts the slabs up to conveyor 102 from which they are removed for stacking on a pallet.
Different types of chop mechanisms 110 exist. One common type of chop mechanism employs a drop (or snail) cam. An example of a chop mechanism 210 that employs a drop cam is shown in FIGS. 2A-2D. Chop mechanism 210 includes a chop frame 212 which houses a blade (not shown). Chop frame 212 is configured to slide within a fixed frame 211 between a loaded (or upward) position and a chopping (or downward) position as illustrated by the sequence in FIGS. 2A-2D. Springs 215 are used to load chop frame 212 for a chopping action.
To cause chop frame 212 to perform this chopping action, cam 213 and follower 214 are employed. Follower 214 is mounted to chop frame 212 in a position that causes the follower to be lifted up as cam 213 rotates in a clockwise direction. Lifting chop frame 212 compresses springs 215 thereby loading the chop frame for the chopping action. Due to the flat edge 213a (or snail shape) of cam 213, follower 214 will be dropped after it has been lifted to its highest position. The blade of chop frame 212 will therefore be thrust into the ground once follower 214 is dropped due to the force caused by springs 215 as they return to their unloaded position. After chop frame 212 has descended into the ground, the rotation of cam 213 lifts follower 214 to apply an upward force to withdraw the blade from the ground thereby allowing the sod harvester to continue moving forward during the chopping action.
FIGS. 2A-2D illustrate a sequence of positions through which chop mechanism 210 travels during the chopping action. In FIG. 2A, cam 213 is shown in a ready (or loaded) position. Cam 213 has reached this ready position by rotating clockwise until follower 214 is adjacent to flat edge 213a. In the ready position, chop frame 212 is elevated thereby loading springs 215 and preparing the chop frame to descend into the ground. Although not shown, a latch is typically used to prevent cam 213 from rotating in a reverse direction (i.e., counter-clockwise) once it has reached this ready position.
In some designs, chop mechanism 210 can include means for identifying a distance that the sod harvester has travelled. In such cases, these means can control the advancement of cam 213 from the ready position shown in FIG. 2A to the chopping position shown in FIG. 2B. For example, a sensor may be employed to cause cam 213 to commence rotating from the ready position when it is determined that the sod harvester has travelled a distance equal to the desired length or width of a slab.
Cam 213 can continue to rotate to cause follower 214 to be lifted back towards the ready position as is shown in FIG. 2C. This rotation will cause cam 213 to again reach the ready position as shown in FIG. 2D.
The timing at which cam 213 rotates to the “drop point” (i.e., the position at which cam 213 no longer supports follower 214) determines the length/width of the slabs of sod. Typically, it is desirable that the slabs have a uniform length/width, and therefore, this timing is important. However, it can be very difficult to accurately and repeatedly control the rotation of cam 213 to the drop point.
In particular, a hydraulic motor is typically employed to drive cam 213. With hydraulic motors, there is a delay between the moment when the motor is turned on and the moment when the cam commences rotating. The length of this delay is influenced by various factors including the amount of time it takes to open a valve to allow fluid to commence flowing, and the amount of time it takes for the fluid pressure to build to a point that it exceeds the resting inertia of the cam. Another factor that influences the timing is the amount of time it takes for the hydraulic motor to reach full speed. In other words, once the motor is turned on, the hydraulic pressure will increase at some rate. Accordingly, when designing a control unit for controlling the rotation of cam 213, the designer must account for these delays so that the motor can be turned on in anticipation of the sod harvester reaching the location where a chopping action should be performed.
In addition to the timing for moving from the ready position to the drop point, the timing for reaching the ready position is also important. As with turning the motor on, there are also delays between the time when the motor is turned off and when the cam stops rotating. These delays must be taken into account when rotating the cam to the ready position. An error in this timing can result in the cam overrunning the ready position. If the overrunning is sufficient to rotate the cam to the drop point, a chopping action will occur too soon resulting in the slab being too short. On the other hand, if the overrunning does not cause the cam to reach the drop point, the cam will reverse direction due to the loaded springs until it slams against the latch. This can lead to early latch failure and increased wear on the cam and other components of the chop mechanism.
In contrast, if the motor is turned off too soon so that the cam does not reach the ready state (or at least the point where the latch engages to prevent reverse rotation), the cam may be allowed to rotate freely in the reverse direction until the springs are unloaded.
To address these timing issues, a controller can be used to control the rotation of the cam. Such controllers are typically programmed to employ timing offsets to account for the delays inherent in a hydraulic system. However, even when employing a controller, it can still be difficult to achieve uniform slab lengths/widths. A primary reason for this difficulty is that the delays inherent in such chop mechanisms are dependent on operating and environmental conditions. For example, as the hydraulic fluid's temperature increases, its density and viscosity decrease. Therefore, the delays present when turning the motor on or off vary with the temperature of the hydraulic fluid. Similarly, wear on the hydraulic components can alter the pressures of the hydraulic fluid resulting in changes in the delays over time.
When the ground speed of the sod harvester is slow, the variations in the timing delays are oftentimes inconsequential. For example, when the sod harvester is moving slowly, a slightly increased delay in reaching the drop point will not cause the slab to be substantially longer than expected. In contrast, if the sod harvester is operated at a relatively fast ground speed, a seemingly insignificant variation in the timing of reaching the drop point may result in the slab having a width/length that unsatisfactorily exceeds the dimensions of the pallet.
Further, even without such variations, these timing delays can limit the rate at which a sod harvester can be operated. The frequency of the chopping action is dependent on the ground speed of the sod harvester and the desired length/width of the slab. This frequency is limited by the delays of starting and stopping the motor. In particular, the frequency of the chopping action is limited by the amount of time it takes to rotate the cam from the ready position, to the drop position, and then back to the ready position. As stated above, there is a first delay when commencing the rotation from the ready position and a second delay when stopping the rotation to again reach the ready position. The combination of these delays, along with the actual time required to rotate the cam, limit the harvesting rate.
In summary, with existing chop mechanism designs that employ hydraulic motors, it is very difficult to achieve precise timing of the chopping action, and this difficulty increases as the rate of the chopping action increases. This difficulty is a primary limitation to the rate at which sod can be harvested.
Accordingly, there is a need for a chop mechanism design that is simple yet capable of being operated at high rates. In particular, there is a need for a chop mechanism design that would allow a sod harvester to be operated at high ground speeds while cutting slabs of relatively short length/width in a precise manner.